Wet Swale
A wet swale does not reduce stormwater volume. While this BMP does not provide stormwater volume
reduction towards the performance goal or on an annual basis, it does provide annual pollutant load
reductions. If runoff to the wet swale flows over a side slope through sheet flow, a swale side slope
BMP should be used in combination with the wet swale BMP.
MIDS calculator user inputs for wet swale
For the wet swale BMP, BMP watershed areas is the only required design parameter.
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Watershed tab
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BMP Name: this cell is auto-filled but can be changed by the user.
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Routing/downstream BMP: if this BMP is part of a treatment train and water is being
routed from this BMP to another BMP, the user selects the name of the BMP from the
dropdown box to which water is being routed. All water must be routed to a single
downstream BMP. Note that the user must include the BMP receiving the routed water
in the Schematic or the BMP will not appear in the dropdown box.
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BMP Watershed Area: BMP watershed areas are the areas draining directly to the BMP.
Values can be added for four soil types (Hydrologic Soil Groups (HSG) A, B, C, D) and for
three Land Cover types (Forest/Open Space, Managed Turf and impervious). The
surface area of the BMP should be included as a managed turf land cover under the
hydrologic soils group of the native soils located under the BMP. Units are in acres.
BMP Summary Tab: The BMP Summary tab summarizes the volume and pollutant reductions
provided by the specific BMP. It details the performance goal volume reductions and annual
average volume, dissolved P, particulate P, and TSS load reductions. Included in the summary
are the total volume and pollutant loads received by the BMP from its direct watershed, from
upstream BMPs and a combined value of the two. Also included in the summary, are the volume
and pollutant load reductions provided by the BMP, in addition to the volume and pollutant
loads that exit the BMP through the outflow. This outflow load and volume is what is routed to
the downstream BMP if one is defined in the Watershed tab. Finally, percent reductions are
provided for the percent of the performance goal achieved, percent annual runoff volume
retained, total percent annual particulate phosphorus reduction, total percent annual dissolved
phosphorus reduction, total percent annual TP reduction, and total percent annual TSS
reduction.
Methodology
Required Treatment Volume
“Required treatment volume,” or the volume of stormwater runoff delivered to the BMP, equals the
performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to
the BMP. This stormwater is delivered to the BMP instantaneously following the Kerplunk method.
Volume Reduction
This BMP does not provide volume reduction towards the performance goal.
Pollutant Reduction
Pollutant load reductions are calculated on an annual basis. A wet swale does not provide annual
volume reduction. Fixed removal rates of 68 percent TSS, 0 percent particulate phosphorus, and 0
percent dissolved phosphorus are applied to water that travels through the wet swale.
NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the
calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3
milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if
the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant
reduced.
Routing
A wet swale can be routed to any other BMP except for a green roof and a swale side slope or any BMP
that would cause water to be rerouted back to the wet swale. All BMPs can be routed to a wet swale.
Assumptions for swale main channel
The following general assumption applies in calculating the credits for a wet swale. If this assumption is
not followed the pollutant reduction credits cannot be applied.
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The wet swale has been properly designed, constructed and will be properly maintained
according to specifications for filtration systems.
On the watershed tab, the following warning is given:
“The calculator does not require sizing inputs for non-volume reducing BMPs. This BMP should be sized
according to the guidelines in the MN stormwater manual.” Design and construction criteria can be
found at this link.
Images
Symbol for wet
swale in MIDS
calculator
BMP watershed area parameters